In high-energy physics it is known to use cassette-shaped scintillation cells, which for example consist of 5×5 cm discs with a thickness of 0.5 cm and which are provided with a circular groove. A wavelength shifting fibre is inserted into the groove, whose end terminates in a photomultiplier. Wavelength shifting fibres are synthetic fibres which produce a shifting of the wavelength. Light entering, in particular light entering via the longitudinal side, generates new light entering via the longitudinal side, generates new light flashes in the fibre. The newly generated light flashes spread within the fibre and reach its end from where they are passed to the photomultiplier.